Dibenzeneborinic acid

Phenylboric acid (benzeneboronic acid) [98-80-6] M 121.9, m -43", 215-216" (anhydride), 217-220", pKj 8.83. It recrystallises from H2O, but can convert spontaneously to benzeneboronic anhydride or phenylboroxide on standing in dry air. Possible impurity is dibenzeneborinic acid which can be removed by washing with pet ether. Heating in an oven at 110°/760mm Ih converts it to the anhydride m 214-216°. Its solubility in H2O is 1.1% at 0° and 2.5% at 25° and in EtOH it is 10% (w/v). [Gilman and Moore J Am Chem Soc 80 3609 1958.] If the acid is required, not the anhydride, the acid (from recrystallisation in H2O) is dried in a slow stream of air saturated with H2O. The anhydride is converted to the acid by recrystallisation from H2O. The acid gradually dehydrates to the anhydride if left in air at room temperature with 30-40% relative humidity. The melting point is usually that of the anhydride because the acid dehydrates before it melts [Washburn et al. Org Synth Coll Vol IV 68 7965]. 

While stirring is continued, the aqueous distilland is cooled in an ice bath (Note 10). The benzeneboronic acid, which separates as small white crystals, is collected on a Buchner funnel and washed with petroleum ether. The petroleum ether removes traces of dibenzeneborinic acid, which are seen in the hot mother liquor as globules of brown oil and which may color the product. The acid is dehydrated to benzeneboronic anhydride by heating it in an oven at 110° and atmospheric pressure for 6 hours (Note 11). Benzeneboronic anhydride is obtained as a colorless solid, weight 240-247 g. (77-79%) (Note 12), m.p. 214-216°. 

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